1. Field of the Invention
The present invention relates to a free oscillating current supply for a monitor for the screen of a communications terminal or the like, having a clock-controlled voltage converter which, from a commercial AC voltage supply, generates at least one DC voltage which is suitable for operation of the screen, and having a deflecting device assigned to the screen and having a predetermined line frequency.
2. Description of the Prior Art
In order to convert commercial AC voltage to DC voltages required for the operation of video display units, monitors and the like, it has been the practice for some time to use switched power supplies. Such switched power supplies combine the advantages of a very compact construction coupled with a high-level of control accuracy and efficiency.
The construction and operation of switched power supplies of this type are generally known and described in detail, for example, in the book "Switch-Mode Power Supply Units: Fundamentals Design, Circuitry Examples" by Joachim Wustehube, published by the Expert Verlag, 1979.
In the switched power supply units, the commercial voltage is first rectified and filtered. The D.C. voltage obtained in this manner is fed to a regulated D.C. converter. An electronic switch, in combination with a transformer, secondary rectifier and filter circuit, and with a magnetic store and a charging capacitor, facilitates electrically-isolated transmission with low losses. The mean value of the output voltage can be influenced by periodically opening and closing the electronic switch and by changing the ratio of on time to period duration (pulse duty factor). When switched power supplies of this type are used for the current supply of monitors, interference between the current supply switching frequency and the line frequency of the monitor results in picture interference if the switching frequency is not precisely synchronized with the line frequency.
In addition, the pick up of magnetic fields of the transformer causes the electron beam to be modulated in intensity and the characters on the screen to change in brightness, resulting in flickering of the characters. In particular, in many applications the influence of ripples in the current supply produces problems in respect of the monitor picture. Small, superimposed interferences in the supply voltage become visible in the picture as flickering, strip formation or snow. Effects of this kind are disturbing to the observer and must be avoided in the interest of good picture quality.
A known solution is to synchronize the switching frequency of the converter with the line frequency of the monitor.
Therefore, the German allowed and published application No. 1,294,527 discloses a method of stabilizing a supply voltage for a load with its own clock frequency, using a switching transistor which serves as a control component in the series arm of the stabilizing circuit, a choke coil which serves as an energy store, and a diode which is arranged in the shunt arm and which has the reverse polarity to the operating voltage. Here, the switching transistor is controlled by a rectangular control voltage which has a constant period duration and a pulse duty factor which is dependent upon the output voltage to be stabilized, so that the frequency of the rectangular control voltage is automatically adjusted to the clock frequency or a whole numbered multiple of the clock frequency of the load as soon as the latter is switched on. A disadvantage of this method is that the clock frequency emitted from the load must first be prepared via a push-pull amplifier, which in turn contains two transformers, before it is supplied to a control component. These transformers entail increased costs, additional space requirements and a higher power loss of the overall switching regulator.
The German Pat. No. 25 27 112 also discloses a switched power supply for a video display unit (VDU) which contains an oscillator which is provided with a control input and the signals of which determine the switching frequency of the switched power supply. The VDU contains a clock pulse generator which generates clock pulses having a predetermined repetition frequency and a predetermined duration in order to construct a picture on the screen on the VDU. Here, the oscillator is connected via an isolating stage to the clock pulse generators so that the repetition frequency of the signals emitted from the oscillator is synchronized with the repetition frequency of the clock pulses emitted from the clock pulse generator. This avoids changes in timing both of the position and the brightness of the characters on the screen which occur in the event of slight differences between the repetition frequency of the clock pulses and the switching frequency of the switching transistor as a result of beating.
This method of synchronizing the line frequency of the monitor with the switching frequency of the clock-controlled monitor current supply has the disadvantage that the expense in terms of components and the associated costs are relatively-high. Therefore, the clock pulse generator must additionally contain a frequency divider which, from the signals for constructing a picture on the screen, produces clock pulses for synchronizing the power supply unit. Apart from the oscillator, an additional isolating stage is required, e.g. an optical coupling element or transformer.
This is technically and financially expensive as a method of achieving synchronization between the switching frequency of the converter and the line frequency of the monitor rules out the possibility of using cost-effective, free oscillating converters.